1. Field of the Disclosure
Embodiments disclosed herein relate generally to the alkylation of propylene with isobutane. More particularly, embodiments disclosed herein relate to a process for the alkylation of isobutane with a dilute propylene stream under conditions to promote the propylene alkylation reaction and to avoid excessive oligomerization or polymerization of propylene.
2. Background
Alkylation is the reaction of paraffins, usually isoparaffins, with an olefin in the presence of a strong acid which produces paraffins, e.g., of higher octane number than the starting materials and which boils in the range of gasolines. In petroleum refining, the alkylation reaction is generally the reaction of a C3 to C5 olefin with isobutane.
In refining alkylations, hydrofluoric or sulfuric acid catalysts are commonly used. For sulfuric acid catalyzed alkylation, low temperature or cold acid processes are favored, minimizing side reactions. In a typical process, the reaction is carried out in a reactor where the hydrocarbon reactants are dispersed into a continuous acid phase.
For example, U.S. Pat. No. 2,762,853 discloses an alkylation process including feeding isoparaffins, such as isobutane or isopentane and C2-C5 monoolefins to an alkylation reactor. The alkylation reaction is catalyzed with sulfuric acid in excess of 88 percent, preferably in excess of 96 percent. The alkylation products are then separated into gasoline range components and heavier alkylate products, among other finishing processes.
As another example, U.S. Pat. No. 2,859,260 discloses an alkylation process including reacting isoparaffins with olefins in the presence of a sulfuric acid catalyst. The reaction product is then separated to recover a hydrocarbon-rich phase and an acid-rich phase. The hydrocarbon-rich phase is further treated to remove catalyst esters from the hydrocarbon phase, among other downstream operations. Another example of a prior art alkylation process is disclosed in U.S. Pat. No. 3,013,092.
Whereas the above alkylation reactions may occur in a single reactor, Albright et al. disclose a two-step alkylation process in which butyl sulfates or butyl fluorides are formed in the first step and alkylate is produced in the second step. See, for example, “Alkylation of Isobutane with C4 Olefins. 1. First-Step Reactions Using Sulfuric Acid Catalyst,” Lyle F. Albright et al., Ind. Eng. Chem. Res. 1988, 27, 381-386 and “Alkylation of Isobutane with C4 Olefins. 3. Two-Step Process Using Sulfuric Acid Catalyst,” Lyle F. Albright et al., Ind. Eng. Chem. Res. 1988, 27, 391-397.
In a modern refinery, one of the gasoline blending components comes from the FCC unit. This FCC unit also produces mixed C4's (butenes/butanes) and mixed C3's (propylene/propane). These light gasses are not suitable as gasoline, so they must be converted into gasoline components or converted into other useful products. In the sulfuric acid alkylation process, as discussed above, C4 olefins (butenes) are reacted with isobutane to produce a mixture of C6 to C9 paraffins. Because the alkylation process produces branched paraffins, the resulting octane value of the product is good. The disadvantages of the process include high energy use, and the need to regenerate and recycle the sulfuric acid due to the buildup of heavy compounds that are soluble in the acid, commonly referred to as “acid soluble oils,” or ASO.
Normally, in a refinery, very few of the C3 compounds from the FCC unit are fed to the alkylation unit. The primary reason is that the reactive propylene is very stable once absorbed in the acid and alkylates slowly. Thus, one problem with conventional sulfuric-acid-catalyzed alkylation using propylene is that the stable absorbed propylene reacts slowly with isobutane. As a result, the propylene tends to form heavy compounds which necessitate additional acid regeneration.
The most prevalent process for conversion of propylene is commonly referred to as “poly.” In a poly unit, the propylene is oligomerized over solid phosphoric acid (SPA) catalyst to form C6 and C9 olefins. These heavier olefins can then be used in gasoline. The non-reactive propane is then sold as a separate fuel. The SPA catalyst life is short and as the reaction gives off heat, the reactor consists of tubes and must be water cooled. In addition, the catalyst is often difficult to remove from the reactor after it is spent, and must sometimes be drilled out due to polymer formation. While this is expensive and maintenance intensive, the refiner has few other economic alternatives for dealing with propylene.
As another alternative, propylene may be separated from propane by distillation and sold as a chemical product. This option is not available to all refiners, as some do not have a nearby customer for the propylene. Consequently, these “stranded” refiners have no choice but to convert the propylene into gasoline range components. The present invention is a new
Accordingly, there exists a need for alternative processes to convert propylene into gasoline in a way that produces more gasoline than oligomerization in a poly unit.